The disclosed invention is generally directed to rectifying power supplies, and is more particularly directed to a rectifying power supply which includes circuitry for reducing losses in the operation of the power rectifying diodes.
Rectifying power supplies are utilized in certain applications where the required supply voltage is DC. The originating power source may provide an AC voltage or a DC voltage. With a DC voltage supply, stepping the voltage up or down requires conversion of the DC power to AC power, which may be accomplished, for example, with a square wave converter or a sine wave converter. The AC power is then stepped up or stepped down as required, and then rectified and filtered.
The rectifying diodes used in rectifying power supplies continuously switch on and off in response to the AC power. As is well known, a diode does not instantaneously turn off, instead providing reverse current pursuant to reverse bias voltage to discharge stored charge. The resulting turn off or "recovery"reverse currents result in equivalent I.sup.2 R power dissipation in the diodes, and current surges or spikes at the inputs to the rectifiers. The current surges or spikes reflect back to the AC power source as a heavier load, which causes higher current stresses on the semiconductor switching devices of the AC power source.
It has been determined that power dissipation and the magnitudes of the current surges or spikes due to diode recovery increase with increased forward current, and also increase with increased AC operating frequency. With increased AC operating frequencies, the voltage slew rate increases and reverse bias voltages increase more rapidly, which results in greater peak reverse currents.
Diode recovery time power dissipation and current surges or spikes are important considerations in high frequency power supplies. While significant improvements in the size and weight of rectifying power supplies have been made by increasing the operating frequency of the AC power, the recovery characteristics of known rectifier diodes results in reduced efficiency and increased component stress at higher frequencies. At some point, the rectifying diodes simply fail.